A Schottky barrier diode is an unipolar device in which electrons serve as the main charge carriers for transporting current. The device has a low forward voltage drop and a fast switching. However, the leakage current of Schottky diodes increases as reverse bias increases because of the lowering of Schottky barrier under high electric field. To reduce the leakage current at the reverse bias, a high work function metal is usually used to provide a high Schottky barrier, which will in turn increase the forward voltage drop and turn-on power loss of the device. The Schottky diode with a trench structure is one of solutions proposed to compromise above mentioned trade-offs. A trench-type Schottky diode usually comprises a plurality of mesas separated by a plurality of trenches. A Schottky contact with a lower barrier formed on the mesa provides a low forward voltage drop, while a metal-oxide-semiconductor (MOS) structure (the trench MOS controlled barrier Schottky diode, TMBS) or a Schottky contact with a higher barrier (the Schottky controlled barrier Schottky diode, TSBS) formed in the trenches shield the electric field on the low barrier contact and thus reduces the leakage current at the reverse bias.
FIG. 1 shows a schematic diagram of a TSBS diode. A trench 101 is formed at the semiconductor substrate 10, wherein a low Schottky barrier is formed on the mesa 103 by a contact metal 12 with a lower work function. Typically, a semiconductor substrate comprises a highly doped layer (as a cathode region) and a drift layer (not shown in FIG. 1), wherein the highly doped layer could be doped to a uniform first conductive type (such as n-type) dopant with concentration of about 1×1019 cm−3, and the drift layer (provided by epitaxial fabrication) may have a carrier concentration of 1×1015 cm−3˜1×1017 cm−3. A high Schottky barrier is formed in the trench 101 by a contact metal 14 with a higher work function. The TSBS diode utilizes the higher barrier Schottky contact within the trenches to shield the high electric field generated in the semiconductor drift region from the low barrier Schottky contact to reduce the leakage current at reverse bias.
FIG. 2 shows a schematic diagram of a TMBS diode. An insulating layer 22 and a metal layer 24 are formed in the trench 201 of the semiconductor substrate 20. A low Schottky barrier is formed on the mesa 203 by a contact metal 26 with a low work function. The metal layer 24, the insulating layer 22 and semiconductor of the substrate 20 form a metal-oxide-semiconductor (MOS) structure within the trench 201, the TMBS diode utilizes the depletion region formed by the MOS structure to shield the high electric field generated in the semiconductor drift region from the low barrier Schottky contact to reduce the leakage current at reverse bias. A thin insulating layer 22 is required to effectively generate the depletion region under the trench MOS structure at reverse bias, whereas a thick insulating layer 22 is formed to sustain the high electric field at high reverse bias because some of the applied reverse bias is supported across the oxide. When a TMBS device is made of silicon, the above mentioned problem is relatively minor because the breakdown strength of oxide is much larger than that of silicon (the breakdown strength of silicon is about 0.3 MV/cm, where the breakdown strength of silicon oxide (SiO2) is around 8˜10 MV/cm). However, when a TMBS device is made of a wide bandgap semiconductor material, such as silicon carbide (SiC), the reliability issue arose, because the breakdown strength of SiC (about 3 MV/cm) is on the same order of magnitude to that of SiO2. If a thick insulating layer is used in the TMBS device to improve the reliability, the capability of effectively generating depletion region of the trench MOS structure deteriorated and the leakage current increased.
Although there is no hard breakdown for the TSBS of FIG. 1 as what may happen in the TMBS, the highest barrier Schottky contact achievable within the trenches limits the application of TSBS for high voltage rating SiC devices because of the barrier lowering effect caused by image force when a high electric field crowded around the trench corners.
Therefore, the silicon carbide Schottky diodes with voltage ratings higher than 600V mainly adopt junction barrier Schottky (JBS) structures. In the JBS structure, pn-junctions formed by spaced p+ regions implanted on the n-type silicon carbide drift layer using aluminum are used to generate depletion regions to shield the electric field from the Schottky contact to reduce leakage current under reverse bias. Nevertheless the implantation processes in SiC require an elevated temperature (400˜700° C.) where normal photoresists are not adequate for masking and a hardmask made of oxide for example has to be used. Following the implantation processes, an ultra-high-temperature annealing (1600˜1800° C.) is further necessary to active the dopants. These high temperature processes increase the manufacturing cost of SiC Schottky diodes.